Several methods have been and are being used to analyze gas streams for hydrogen. Among those still in use are gas chromatography, thermal conductivity, differential pressure across a Palladium membrane, catalytic combustion and surface absorption sensors.
Gas chromatography can be very accurate, sensitive, and a selective method of analyzing gas for hydrogen content but is a batch process and real-time continuous data cannot be obtained. Also the hardware associated with such a measuring technique is relatively expensive.
Thermal conductivity techniques for analyzing gas for hydrogen content provide continuous data, but these devices and techniques are not selective and cannot discriminate between gases having similar thermal conductivities, and are affected by all gases in the sample stream. Thus it must be known beforehand that certain gases are not present before such measuring techniques may be utilized.
Catalytic combustion sensors are used to analyze gas for hydrogen content. They are relatively inexpensive, have fast response time, and good sensitivity. Their problem is however also a lack of selectivity as they will detect all combustibles gases, usually with different gain factors, making data from streams with mixed combustibles practically useless.
Differential pressure across a Palladium membrane is a known method of testing for hydrogen content. It is extremely selective for hydrogen, but present forms of the known devices lack sensitivity and have time constants in the order of 3 minutes. Therefore, even though these are continuous measurement instruments, the long time constant nearly precludes them as real-time hydrogen gas analyzers in process applications.
Differential thermocouples are known for use in thermal analysis as evidenced by U.S. Pat. Nos. 3,906,721 and 4,063,898. However the Applicant is unaware of any such prior art devices wherein such known differential thermocouples are respectively coated with an activated Palladium or activated Palladium-Silver alloy catalyst and a non-catalyst to provide a self-powered, highly responsive, inexpensive, and stable hydrogen gas detector ideally suited for process applications.